The transmission of information across a communications channel requires that the user (or source) information be converted to the form of a signal which is compatible with transmission characteristics of the communications channel. Such conversion is in many cases accomplished through modulation of an electrical or optical carrier signal, or both, by the information content itself or by compound modulation--that electrical or optical signal being transmitted over the communications channel. In the case of information in digital form, the "1"s and "0"s representing the data are encoded into a form that can be used to drive, or modulate an aspect of the transmission carrier signal, such as amplitude, frequency, phase, or polarization.
Various modulation techniques for digital data are known in the art, including on-off keying (OOK), intensity modulation (IM), amplitude modulation (AM), frequency modulation (FM), phase modulation (PM) and polarization modulation. The source information may be line coded prior to any modulation. The line coded data becomes the baseband signal modulating the electrical or optical channel carrier. Typically, such line codes map the data value "1" to a high signal value defined by the coding format and the value "0" to a low signal value. While, these line coding formats can include negative (less than zero) signal values for the low signal state, such a negative low signal state cannot be realized for optical transmission systems--the concept of "negative" light not being realizable, at least in practice. Accordingly, as the use of optical transmission media has become increasingly the norm, "unipolar" line codes have been developed in which the low signal state is maintained at zero or a small non-zero signal level.
Two line coding methods of particular interest are designated "unipolar return-to-zero" (URZ) and "unipolar non-return-to-zero" (UNRZ) coding formats. URZ and UNRZ coding are types of on-off-keying modulation well known in the communications arts. UNRZ coding is the common coding technique for transmitting data in optical communications systems.
In optical communications systems, the optical carrier signal is normally provided by a laser light source, such as a laser diode or a light emitting diode (LED). The optical output of the laser diode is typically modulated with the UNRZ-coded baseband signal, representing the coded source information, and this modulated light-wave signal will be transmitted across an optical transmission medium, such as a fiber-optic cable or free space. Modulation of the laser output may be either direct, by varying the laser diode current in proportion to the modulating signal, or indirect, through use of an external modulator. At the receiver end of the optical transmission path, an optical demodulator is applied to recover the coded baseband signal, which signal is then decoded to recover the transmitted source information.
When a laser diode is directly modulated, i.e., the laser diode current is changed in accordance with the digital data to be transmitted, random changes occur in the optical power output (RIN) and in the optical frequency, producing what is termed chirp. As is understood by those skilled in the art, chirp broadens the output spectrum of the laser diode. That is, the linewidth of the laser diode is broadened and thus additional optical frequencies are present at the transmitter within the information signal bandwidth. As a result of chirp, transmission distances are shortened and data rates are reduced. Furthermore, optical power limitations may be imposed to significantly limit energy utilized as a result of the additional frequencies which are present because of chirp. As will also be well understood, the degree of chirp experienced is dependent on modulation waveform types, such as, square wave (maximum chirp), triangular wave, and sinusoidal wave (minimum chirp).
The output signal from laser diodes will often include one or more of the following noise components, even without data modulation being applied: 1) random optical amplitude fluctuations, referred to as relative intensity noise (RIN) amplitude; 2) random optical phase fluctuations, where the optical phase noise may also be related to optical frequency noise; and 3) random polarization fluctuations which result because of random polarization phase changes or individual random amplitude changes of polarization states or both.
In addition to the above-noted limitations which are present with a laser diode used in connection with high speed optical communications, the fiber optic cable as used in the transmission introduces certain impairments into the data due to the non-linearities in the fiber optic cable itself. As would be understood, the impairments may include dispersion, self phase modulation (SPM), cross phase modulation (XPM), etc.
For short-haul, low bandwidth applications, where chirp is normally not a substantial problem, direct intensity modulation of the laser diode is used. For the case of high bit-rate data and/or long haul transmission, however, an external modulator will normally be used to modulate the light carrier signal from the laser diode.
With optical communications systems, random changes in carrier signal phase and random fluctuations of the light polarization in that signal lead to a reduced signal-to-noise ratio (SNR) at the receiver. It is also to be noted that, for the case of long-haul optical transmission systems (as to which external modulation is normally applied) optical amplifiers may also be required to periodically boost the signal level. The fiber optic cable and optical amplifiers are also sensitive to polarization fluctuations.
In the prior art, an effort has been made to reduce the impact of these polarization and phase related noise factors by modulating the polarization and phase signals with the system clock (or a multiple or sub-multiple thereof). This effort has met with decidedly mixed results. In particular, with clock modulation of phase and/or polarization signals, the SNR of the optical signal shows an unstable improvement. It is the inventor's understanding that these prior-art techniques offer some improvement for certain data patterns and a degradation for other data patterns. Among the problems known to occur with such clock modulation of phase and polarization signals, the clock delay and power penalty or gain is dependent on a wavelength and data pattern. For wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) systems, this is a very expensive and time consuming method to implement. Any perturbation from neighboring channels also poses problems in optimization because optical amplifier gain is pattern dependent when OOK modulation is used. Moreover, the harmonics of the clock generated signal components are hard to filter and are a source of unwanted interference at the transmitter, the receiver and in the transmission channel.
Accordingly, there is a need in the art for an optical transmission system which provides an improved method for reducing unwanted signal generation with the use of system clock (or multiples/sub-multiples) and the interference to a data signal resulting therefrom. There is also a need to obtain a dependable and stable improvement in such unwanted interfering signals across the transmitter, optical transmission channel, optical amplifier(s) and receiver.